The invention relates to a digital signal transmission system operating with constellation coding, the system comprising an orthogonal frequency-division multiplexing (OFDM) modulation unit which generates multicarrier signals, and an amplifier which causes signal distortion.
The digital transmission system can be used for transmitting digital television signals (audio and/or video) to portable receivers, mobile receivers or others.
The document "Principles of modulation and channel coding for digital broadcasting for mobile receivers" by N. Alard and R. Lasalle, EBU Review-Technical, No. 224, August 1987, Brussels (Belgium), is known. This document describes the orthogonal frequency-division multiplexing modulation technique which consists of frequency-division multiplexing various orthogonally modulated carriers, for example, by symbols from a constellation. Such a technique is very robust with respect to fading of the received signal when the transmission channel is modified.
The multicarrier modulation technique, for example, implementing Orthogonal Frequency-Division Multiplexing, will hereinafter be referred to as OFDM.
When the OFDM signals are formed, they are to be transmitted through a channel and are therefore to be amplified by a power amplifier. A particularly critical point for microwave links and transmission either by satellite or groundbased channel is the non-linearity of the transmitter power amplifier or of the on-board power amplifier in the case of satellite transmissions. These amplifiers are known for their non-linear characteristics. If they are used in their linear zones, their power is not completely employed. If they are made to operate near their saturation power level, they unacceptably distort the signal. In practice, with a given power amplifier, the level of the transmitted signal is fixed to establish a compromise between the signal-to-noise ratio and the non-linear distortion the signal is subjected to. Thus, the optimum operating point of the amplifier is the one at which the compound effects of additive noise of the channel and non-linear distortion of the amplifier are minimized. For modulations with a large number of states this point lies far from the saturation power level of the amplifier, which means that the latter is not used efficiently. In the case of a monocarrier system, predistortion techniques (fixed or adaptive) are currently used to increase the system efficiency, which techniques make it possible to reduce the effect of the non-linearity of the power amplifier on the transmitted signal.
In the case of a monocarrier system, a known technique consists of modifying the alphabet of transmit data. This technique called "data predistortion technique" or "baseband predistortion technique" is known from the article by A. A. M. Saleh and J. Salz "Adaptive linearization of power amplifiers in digital radio systems", Bell System Technical Journal, Volume 62, April 1993, pages 1019/1033. In the case of modulations with a large number of states, for example, a two-carrier quadrature amplitude modulation (QAM64, QAM256), the amplifier influences the constellation by producing a net compression and a net rotation of the points having a large amplitude. To compensate for this effect, the original constellation is distorted in such a way that it returns to its original square shape after its passing through the power amplifier.
Such a predistortion technique which influences a monocarrier system cannot be applied to a multicarrier system. In effect, with a monocarrier system the predistortion is effected in the symbols of the constellation. If the same technique is applied to each of the carriers of an OFDM system, distortions due to intercarrier interference which cannot be corrected will appear on the output of the amplifier. This intercarrier interference is created between the transmitted symbols by other carriers. On the other hand, a multicarrier system is much more sensitive than a monocarrier system to non-linearities of the amplifier, because the peak amplitudes are much larger for OFDM signals than for monocarrier signals.